1. Field of the Invention
The present invention relates to a laminated film and a method for producing the same. More particularly, the present invention relates a laminated film comprising a thin film of YBa.sub.2 Cu.sub.3 O.sub.7-x which is attractive as a superconductor exhibiting superconductivity around 90 K and an ultrathin layer of MgO formed on said superconductor layer and a method for producing the same.
2. Description of the Related Art
Superconductors including YBa.sub.2 Cu.sub.3 O.sub.7-x will find various new applications such as Josephson tunnelint junctions.
To product the Josephson tunneling junctions, an insulating ultrathin layer having a thickness of 30 .ANG. or less should be formed between the superconductor films for tunnel junction. To form such junction, it is essential to produce a superconductive film having good surface smoothness and an insulating layer on it. A thickness of the insulating ultrathin layer for forming a junction is limited by a coherence length of a superconductor film. The coherent length in the direction perpendicular to the (001) plane is about 4 to 7 .ANG., while that in a direction parallel to said plane is about 15 to 30 .ANG..
Therefore, the thickness of the insulating ultrathin layer to be used for junction varies with a kind of the superconductor and its crystal direction to be connected. When a direction perpendicular to the (001) plane of the superconductor coincides with the direction perpendicular the surface of the insulating layer, the thickness of the latter should be 10 .ANG. or less. On the contrary, when a direction parallel with the (001) plane of the superconductor coincides with the direction perpendicular to the surface of the insulating layer, the thickness of the latter can be as thick as several ten .ANG., which makes the formation of tunnel junction easy.
A thin film of single crystal YBa.sub.2 Cu.sub.3 O.sub.7-x having the (001) plane in a direction parallel with the film surface and its production are disclosed in co-pending U.S. Patent Application No. 07/247,261 filed on Sept. 21, 1989 and EP-A 0 308 869.
The study on epitaxial growth of (110) MgO on (110) NdBaCuO by sputtering was reported in the Preprint for 1988 Autumn 49th Meeting of the Society of Applied Physics, Japan, Vol. 1, page 106.
However, it is very difficult to form the insulating ultrathin layer on the high temperature superconductor film and it has not been possible to form a continuous insulating thin film having a thickness of several ten .ANG..
One of the reasons for the difficulty in the formation of continuous insulating thin film may be that the high temperature superconductor film as a substrate is hardly in the single crystal form and then it cannot be expected to epitaxially grow the insulating ultrathin layer on such superconductor film.
Various studies have been made on the formation of the insulating ultrathin film on the high temperature superconductor films but no continuous insulating ultrathin film has been formed.
Including the case of the above described epitaxial growth of (110) MgO on (110) NdBaCuO by spattering, the epitaxial growth of the crystal is realized in a small area but the grown single crystals scatter on the substrate, so that adjacent peripheral parts of the single crystals are not continuously connected each other as the thickness of the insulating layer is reduced.